Typically, a conventional furnace for continuous annealing of metal strips, such as cold rolled steel strips, is so constructed that such a strip is unreeled from a payoff reel and is introduced into the furnace via a cleaning tank and/or a burn-off chamber which preheats the strip, and in which rolling oils are removed. Thus, the strip is clean when entering the furnace via entry seal rolls located just ahead of a heating chamber. The furnace is provided with a plurality of rolls, which guide the strip through the furnace, as the strip is subjected to heating, holding, slow cooling and fast cooling. The heating temperatures and heating rate in the heating chamber, the holding time in a holding chamber, and the cooling rate and temperature in a cooling chamber are dependent upon the mechanical properties desired for the end product. Another type of strip annealing furnaces includes a heating section, a water quench, a reheating drawing section and a cooling section.
After a strip has been suitably annealed so as to achieve high tensile strength or other mechanical properties that may be desired, the strip may be used as is or may be further processed. Thus, for example, the annealed strip may be tin-plated in a separate, continuous, tin-plating line or galvanized in a zinc pot in line with the annealing furnace.
In the heating chamber of such a furnace, the strip is typically heated by radiant energy from radiant tubes. After leaving the heating chamber, the strip is held for the desired period of time at the required annealing temperature in a holding chamber with radiant tubes. After leaving the holding chamber, the strip is cooled in a cooling chamber, in which the strip may be slow cooled at a controlled rate by air tubes and then fast cooled by fast jet coolers. The strip is heated and cooled in a protective atmosphere consisting of a mixture of hydrogen and nitrogen with a low dew point to prevent oxidation within the furnace chambers. Typical atmosphere mixtures for carbon steel strip consist of about 5% by volume hydrogen and about 95% by volume nitrogen for steel to be tin plated and of about 25% by volume hydrogen and about 75% by volume nitrogen for steel strip to be galvanized. The heating chamber is divided into discrete zones of control. Each zone has several radiant tubes and a zone thermocouple, which is located between the radiant tubes and the strip. The zone thermocouple coacts with a process controller to adjust the heat output of the radiant tubes.
Typically, the heating zones are maintained at temperatures that are significantly higher than the required final strip temperature. For example, for commercial grades of carbon steel strip to be galvanized, the heating zones are maintained at about 1800.degree. F. for a required final strip temperature of approximately 1280.degree. F. During furnace operation, if the strip speed should slow down, the strip would be heated to a temperature that would be significantly higher than the required final strip temperature. Such overheating would adversely effect the mechanical properties of the strip. For proper mechanical properties, the time that the strip is held in the holding chamber varies with the metallurgical requirements of the final product. In some cases, the strip does not require a holding time. Furthermore, the cooling rates and final strip temperatures where the strip is discharged from the furnace depend upon the mechanical properties required for the final product.